Ultrastructural analysis reveals mitochondrial placement independent of synapse placement in fine caliber C. elegans neurons

Neurons rely on mitochondria for an efficient supply of ATP and other metabolites. However, while neurons are highly elongated, mitochondria are discrete and limited in number. Due to the slow rates of diffusion over long distances it follows that neurons would benefit from an ability to control the distribution of mitochondria to sites of high metabolic activity, such as synapses. It is assumed that neurons' possess this capacity, but ultrastructural data over substantial portions of a neuron's extent that would allow for tests of such hypotheses are scarce. Here, we mined the Caenorhabditis elegans electron micrographs of John White and Sydney Brenner and found systematic differences in average mitochondrial length (ranging from 1.3 to 2.4 μm), volume density (3.7% to 6.5%) and diameter (0.18 to 0.24 μm) between neurons of different neurotransmitter type and function, but found limited differences in mitochondrial morphometrics between axons and dendrites of the same neurons. Analyses of distance intervals found mitochondria to be distributed randomly with respect to presynaptic specializations, and an indication that mitochondria were displaced from postsynaptic specializations. Presynaptic specializations were primarily localized to varicosities, but mitochondria were no more likely to be found in synaptic varicosities than non-synaptic varicosities. Consistently, mitochondrial volume density was no greater in varicosities with synapses. Therefore, beyond the capacity to disperse mitochondria throughout their length, at least in C. elegans, fine caliber neurons manifest limited sub-cellular control of mitochondrial size and distribution.

Mitochondrial phosphagen kinases support the volatile power demands of motor nerve terminals

Motor neurons are the longest neurons in the body, with axon terminals separated from the soma by as much as a meter. These terminals are largely autonomous with regard to their bioenergetic metabolism and must burn energy at a high rate to sustain muscle contraction. Here, through computer simulation and drawing on previously published empirical data, we determined that motor neuron terminals in Drosophila larvae experience highly volatile power demands. It might not be surprising then, that we discovered the mitochondria in the motor neuron terminals of both Drosophila and mice to be heavily decorated with phosphagen kinases - a key element in an energy storage and buffering system well-characterized in fast-twitch muscle fibres. Knockdown of arginine kinase 1 (ArgK1) in Drosophila larval motor neurons led to several bioenergetic deficits, including mitochondrial matrix acidification and a faster decline in the cytosol ATP to ADP ratio during axon burst firing. KEY POINTS: Neurons commonly fire in bursts imposing highly volatile demands on the bioenergetic machinery that generates ATP. Using a computational approach, we built profiles of presynaptic power demand at the level of single action potentials, as well as the transition from rest to sustained activity. Phosphagen systems are known to buffer ATP levels in muscles and we demonstrate that phosphagen kinases, which support such phosphagen systems, also localize to mitochondria in motor nerve terminals of fruit flies and mice. By knocking down phosphagen kinases in fruit fly motor nerve terminals, and using fluorescent reporters of the ATP:ADP ratio, lactate, pH and Ca2+ , we demonstrate a role for phosphagen kinases in stabilizing presynaptic ATP levels. These data indicate that the maintenance of phosphagen systems in motor neurons, and not just muscle, could be a beneficial initiative in sustaining musculoskeletal health and performance.

Physiologic and Nanoscale Distinctions Define Glutamatergic Synapses in Tonic vs Phasic Neurons

Neurons exhibit a striking degree of functional diversity, each one tuned to the needs of the circuitry in which it is embedded. A fundamental functional dichotomy occurs in activity patterns, with some neurons firing at a relatively constant "tonic" rate, while others fire in bursts, a "phasic" pattern. Synapses formed by tonic versus phasic neurons are also functionally differentiated, yet the bases of their distinctive properties remain enigmatic. A major challenge toward illuminating the synaptic differences between tonic and phasic neurons is the difficulty in isolating their physiological properties. At the Drosophila neuromuscular junction, most muscle fibers are coinnervated by two motor neurons: the tonic "MN-Ib" and phasic "MN-Is." Here, we used selective expression of a newly developed botulinum neurotoxin transgene to silence tonic or phasic motor neurons in Drosophila larvae of either sex. This approach highlighted major differences in their neurotransmitter release properties, including probability, short-term plasticity, and vesicle pools. Furthermore, Ca2+ imaging demonstrated ∼2-fold greater Ca2+ influx at phasic neuron release sites relative to tonic, along with an enhanced synaptic vesicle coupling. Finally, confocal and super-resolution imaging revealed that phasic neuron release sites are organized in a more compact arrangement, with enhanced stoichiometry of voltage-gated Ca2+ channels relative to other active zone scaffolds. These data suggest that distinctions in active zone nano-architecture and Ca2+ influx collaborate to differentially tune glutamate release at tonic versus phasic synaptic subtypes.SIGNIFICANCE STATEMENT "Tonic" and "phasic" neuronal subtypes, based on differential firing properties, are common across many nervous systems. Using a recently developed approach to selectively silence transmission from one of these two neurons, we reveal specialized synaptic functional and structural properties that distinguish these specialized neurons. This study provides important insights into how input-specific synaptic diversity is achieved, which could have implications for neurologic disorders that involve changes in synaptic function.

Computational modeling predicts ephemeral acidic microdomains in the glutamatergic synaptic cleft

At chemical synapses, synaptic vesicles release their acidic contents into the cleft, leading to the expectation that the cleft should acidify. However, fluorescent pH probes targeted to the cleft of conventional glutamatergic synapses in both fruit flies and mice reveal cleft alkalinization rather than acidification. Here, using a reaction-diffusion scheme, we modeled pH dynamics at the Drosophila neuromuscular junction as glutamate, ATP, and protons (H⁺) were released into the cleft. The model incorporates bicarbonate and phosphate buffering systems as well as plasma membrane calcium-ATPase activity and predicts substantial cleft acidification but only for fractions of a millisecond after neurotransmitter release. Thereafter, the cleft rapidly alkalinizes and remains alkaline for over 100 ms because the plasma membrane calcium-ATPase removes H⁺ from the cleft in exchange for calcium ions from adjacent pre- and postsynaptic compartments, thus recapitulating the empirical data. The extent of synaptic vesicle loading and time course of exocytosis have little influence on the magnitude of acidification. Phosphate but not bicarbonate buffering is effective at suppressing the magnitude and time course of the acid spike, whereas both buffering systems are effective at suppressing cleft alkalinization. The small volume of the cleft levies a powerful influence on the magnitude of alkalinization and its time course. Structural features that open the cleft to adjacent spaces appear to be essential for alleviating the extent of pH transients accompanying neurotransmission.

Neto-α Controls Synapse Organization and Homeostasis at the Drosophila Neuromuscular Junction

Glutamate receptor auxiliary proteins control receptor distribution and function, ultimately controlling synapse assembly, maturation, and plasticity. At the Drosophila neuromuscular junction (NMJ), a synapse with both pre- and postsynaptic kainate-type glutamate receptors (KARs), we show that the auxiliary protein Neto evolved functionally distinct isoforms to modulate synapse development and homeostasis. Using genetics, cell biology, and electrophysiology, we demonstrate that Neto-α functions on both sides of the NMJ. In muscle, Neto-α limits the size of the postsynaptic receptor field. In motor neurons (MNs), Neto-α controls neurotransmitter release in a KAR-dependent manner. In addition, Neto-α is both required and sufficient for the presynaptic increase in neurotransmitter release in response to reduced postsynaptic sensitivity. This KAR-independent function of Neto-α is involved in activity-induced cytomatrix remodeling. We propose that Drosophila ensures NMJ functionality by acquiring two Neto isoforms with differential expression patterns and activities.

The Krebs Cycle Enzyme Isocitrate Dehydrogenase 3A Couples Mitochondrial Metabolism to Synaptic Transmission

Neurotransmission is a tightly regulated Ca²⁺-dependent process. Upon Ca²⁺ influx, Synaptotagmin1 (Syt1) promotes fusion of synaptic vesicles (SVs) with the plasma membrane. This requires regulation at multiple levels, but the role of metabolites in SV release is unclear. Here, we uncover a role for isocitrate dehydrogenase 3a (idh3a), a Krebs cycle enzyme, in neurotransmission. Loss of idh3a leads to a reduction of the metabolite, alpha-ketoglutarate (αKG), causing defects in synaptic transmission similar to the loss of syt1. Supplementing idh3a flies with αKG suppresses these defects through an ATP or neurotransmitter-independent mechanism. Indeed, αKG, but not glutamate, enhances Syt1-dependent fusion in a reconstitution assay. αKG promotes interaction between the C2-domains of Syt1 and phospholipids. The data reveal conserved metabolic regulation of synaptic transmission via αKG. Our studies provide a synaptic role for αKG, a metabolite that has been proposed as a treatment for aging and neurodegenerative disorders.

Na+ /H+ exchange via the Drosophila vesicular glutamate transporter mediates activity-induced acid efflux from presynaptic terminals

KEY POINTS

Intracellular pH regulation is vital to neurons as nerve activity produces large and rapid acid loads in presynaptic terminals. Rapid clearance of acid loads is necessary to maintain control of neurotransmission, but neuronal acid clearance mechanisms remain poorly understood. Glutamate is loaded into synaptic vesicles via the vesicular glutamate transporter (VGLUT), a mechanism conserved across phyla, and this study reports a previously unknown role for VGLUT as an acid-extruding protein when deposited in the plasmamembrane during exocytosis. The finding was made in Drosophila (fruit fly) larval motor neurons through a combined pharamacological and genetic dissection of presynaptic pH homeostatic mechanisms. A dual role for VGLUT serves to integrate neuronal activity and pH regulation in presynaptic nerve terminals.

ABSTRACT

Neuronal activity can result in transient acidification of presynaptic terminals, and such shifts in cytosolic pH (pH_cyto ) probably influence mechanisms underlying forms of synaptic plasticity with a presynaptic locus. As neuronal activity drives acid loading in presynaptic terminals, we hypothesized that the same activity might drive acid efflux mechanisms to maintain pH_cyto homeostasis. To better understand the integration of neuronal activity and pH_cyto regulation we investigated the acid extrusion mechanisms at Drosophila glutamatergic motorneuron terminals. Expression of a fluorescent genetically encoded pH indicator, named 'pHerry', in the presynaptic cytosol revealed acid efflux following nerve activity to be greater than that predicted from measurements of the intrinsic rate of acid efflux. Analysis of activity-induced acid transients in terminals deficient in either endocytosis or exocytosis revealed an acid efflux mechanism reliant upon synaptic vesicle exocytosis. Pharmacological and genetic dissection in situ and in a heterologous expression system indicate that this acid efflux is mediated by conventional plasmamembrane acid transporters, and also by previously unrecognized intrinsic H⁺ /Na⁺ exchange via the Drosophila vesicular glutamate transporter (DVGLUT). DVGLUT functions not only as a vesicular glutamate transporter but also serves as an acid-extruding protein when deposited on the plasmamembrane.

High-Probability Neurotransmitter Release Sites Represent an Energy-Efficient Design

Nerve terminals contain multiple sites specialized for the release of neurotransmitters. Release usually occurs with low probability, a design thought to confer many advantages. High-probability release sites are not uncommon, but their advantages are not well understood. Here, we test the hypothesis that high-probability release sites represent an energy-efficient design. We examined release site probabilities and energy efficiency at the terminals of two glutamatergic motor neurons synapsing on the same muscle fiber in Drosophila larvae. Through electrophysiological and ultrastructural measurements, we calculated release site probabilities to differ considerably between terminals (0.33 versus 0.11). We estimated the energy required to release and recycle glutamate from the same measurements. The energy required to remove calcium and sodium ions subsequent to nerve excitation was estimated through microfluorimetric and morphological measurements. We calculated energy efficiency as the number of glutamate molecules released per ATP molecule hydrolyzed, and high-probability release site terminals were found to be more efficient (0.13 versus 0.06). Our analytical model indicates that energy efficiency is optimal (∼0.15) at high release site probabilities (∼0.76). As limitations in energy supply constrain neural function, high-probability release sites might ameliorate such constraints by demanding less energy. Energy efficiency can be viewed as one aspect of nerve terminal function, in balance with others, because high-efficiency terminals depress significantly during episodic bursts of activity.

A TRPV channel in Drosophila motor neurons regulates presynaptic resting Ca2+ levels, synapse growth, and synaptic transmission

Presynaptic resting Ca(2+) influences synaptic vesicle (SV) release probability. Here, we report that a TRPV channel, Inactive (Iav), maintains presynaptic resting [Ca(2+)] by promoting Ca(2+) release from the endoplasmic reticulum in Drosophila motor neurons, and is required for both synapse development and neurotransmission. We find that Iav activates the Ca(2+)/calmodulin-dependent protein phosphatase calcineurin, which is essential for presynaptic microtubule stabilization at the neuromuscular junction. Thus, loss of Iav induces destabilization of presynaptic microtubules, resulting in diminished synaptic growth. Interestingly, expression of human TRPV1 in Iav-deficient motor neurons rescues these defects. We also show that the absence of Iav causes lower SV release probability and diminished synaptic transmission, whereas Iav overexpression elevates these synaptic parameters. Together, our findings indicate that Iav acts as a key regulator of synaptic development and function by influencing presynaptic resting [Ca(2+)].

The lack of CuZnSOD leads to impaired neurotransmitter release, neuromuscular junction destabilization and reduced muscle strength in mice

Elevated reactive oxygen species (ROS) production and ROS-dependent protein damage is a common observation in the pathogenesis of many muscle wasting disorders, including sarcopenia. However, the contribution of elevated ROS levels to –a breakdown in neuromuscular communication and muscle atrophy remains unknown. In this study, we examined a copper zinc superoxide dismutase [CuZnSOD (Sod1)] knockout mouse (Sod1^−/−), a mouse model of elevated oxidative stress that exhibits accelerated loss of muscle mass, which recapitulates many phenotypes of sarcopenia as early as 5 months of age. We found that young adult Sod1^−/− mice display a considerable reduction in hind limb skeletal muscle mass and strength when compared to age-matched wild-type mice. These changes are accompanied by gross alterations in neuromuscular junction (NMJ) morphology, including reduced occupancy of the motor endplates by axons, terminal sprouting and axon thinning and irregular swelling. Surprisingly however, the average density of acetylcholine receptors in endplates is preserved. Using in vivo electromyography and ex vivo electrophysiological studies of hind limb muscles in Sod1^−/− mice, we found that motor axons innervating the extensor digitorum longus (EDL) and gastrocnemius muscles release fewer synaptic vesicles upon nerve stimulation. Recordings from individually identified EDL NMJs show that reductions in neurotransmitter release are apparent in the Sod1^−/− mice even when endplates are close to fully innervated. However, electrophysiological properties, such as input resistance, resting membrane potential and spontaneous neurotransmitter release kinetics (but not frequency) are similar between EDL muscles of Sod1^−/− and wild-type mice. Administration of the potassium channel blocker 3,4-diaminopyridine, which broadens the presynaptic action potential, improves both neurotransmitter release and muscle strength. Together, these results suggest that ROS-associated motor nerve terminal dysfunction is a contributor to the observed muscle changes in Sod1^−/− mice.

Mitochondrial free Ca²⁺ levels and their effects on energy metabolism in Drosophila motor nerve terminals

Mitochondrial Ca²⁺ uptake exerts dual effects on mitochondria. Ca²⁺ accumulation in the mitochondrial matrix dissipates membrane potential (ΔΨm), but Ca²⁺ binding of the intramitochondrial enzymes accelerates oxidative phosphorylation, leading to mitochondrial hyperpolarization. The levels of matrix free Ca²⁺ ([Ca²⁺]m) that trigger these metabolic responses in mitochondria in nerve terminals have not been determined. Here, we estimated [Ca²⁺]m in motor neuron terminals of Drosophila larvae using two methods: the relative responses of two chemical Ca²⁺ indicators with a 20-fold difference in Ca²⁺ affinity (rhod-FF and rhod-5N), and the response of a low-affinity, genetically encoded ratiometric Ca²⁺ indicator (D4cpv) calibrated against known Ca²⁺ levels. Matrix pH (pHm) and ΔΨm were monitored using ratiometric pericam and tetramethylrhodamine ethyl ester probe, respectively, to determine when mitochondrial energy metabolism was elevated. At rest, [Ca²⁺]m was 0.22 ± 0.04 μM, but it rose to ~26 μM (24.3 ± 3.4 μM with rhod-FF/rhod-5N and 27.0 ± 2.6 μM with D4cpv) when the axon fired close to its endogenous frequency for only 2 s. This elevation in [Ca²⁺]m coincided with a rapid elevation in pHm and was followed by an after-stimulus ΔΨm hyperpolarization. However, pHm decreased and no ΔΨm hyperpolarization was observed in response to lower levels of [Ca²⁺]m, up to 13.1 μM. These data indicate that surprisingly high levels of [Ca²⁺]m are required to stimulate presynaptic mitochondrial energy metabolism.

Neuron-specific expression of CuZnSOD prevents the loss of muscle mass and function that occurs in homozygous CuZnSOD-knockout mice

Deletion of copper-zinc superoxide dismutase (CuZnSOD) in Sod1(-/-) mice leads to accelerated loss of muscle mass and force during aging, but the losses do not occur with muscle-specific deletion of CuZnSOD. To determine the role of motor neurons in the muscle decline, we generated transgenic Sod1(-/-) mice in which CuZnSOD was expressed under control of the synapsin 1 promoter (SynTgSod1(-/-) mice). SynTgSod1(-/-) mice expressed CuZnSOD in brain, spinal cord, and peripheral nerve, but not in other tissues. Sciatic nerve CuZnSOD content in SynTgSod1(-/-) mice was ~20% that of control mice, but no reduction in muscle mass or isometric force was observed in SynTgSod1(-/-) mice compared with control animals, whereas muscles of age-matched Sod1(-/-) mice displayed 30-40% reductions in mass and force. In addition, increased oxidative damage and adaptations in stress responses observed in muscles of Sod1(-/-) mice were absent in SynTgSod1(-/-) mice, and degeneration of neuromuscular junction (NMJ) structure and function occurred in Sod1(-/-) mice but not in SynTgSod1(-/-) mice. Our data demonstrate that specific CuZnSOD expression in neurons is sufficient to preserve NMJ and skeletal muscle structure and function in Sod1(-/-) mice and suggest that redox homeostasis in motor neurons plays a key role in initiating sarcopenia during aging.

Genetically encoded pH-indicators reveal activity-dependent cytosolic acidification of Drosophila motor nerve termini in vivo

All biochemical processes, including those underlying synaptic function and plasticity, are pH sensitive. Cytosolic pH (pH(cyto)) shifts are known to accompany nerve activity in situ, but technological limitations have prevented characterization of such shifts in vivo. Genetically encoded pH-indicators (GEpHIs) allow for tissue-specific in vivo measurement of pH. We expressed three different GEpHIs in the cytosol of Drosophila larval motor neurons and observed substantial presynaptic acidification in nerve termini during nerve stimulation in situ. SuperEcliptic pHluorin was the most useful GEpHI for studying pH(cyto) shifts in this model system. We determined the resting pH of the nerve terminal cytosol to be 7.30 ± 0.02, and observed a decrease of 0.16 ± 0.01 pH units when the axon was stimulated at 40 Hz for 4 s. Realkalinization occurred upon cessation of stimulation with a time course of 20.54 ± 1.05 s (τ). The chemical pH-indicator 2,7-bis-(2-carboxyethyl)-5-(and-6)-carboxyfluorescein corroborated these changes in pH(cyto). Bicarbonate-derived buffering did not contribute to buffering of acid loads from short (≤ 4 s) trains of action potentials but did buffer slow (~60 s) acid loads. The magnitude of cytosolic acid transients correlated with cytosolic Ca(2+) increase upon stimulation, and partial inhibition of the plasma membrane Ca(2+)-ATPase, a Ca(2+)/H(+) exchanger, attenuated pH(cyto) shifts. Repeated stimulus trains mimicking motor patterns generated greater cytosolic acidification (~0.30 pH units). Imaging through the cuticle of intact larvae revealed spontaneous pH(cyto) shifts in presynaptic termini in vivo, similar to those seen in situ during fictive locomotion, indicating that presynaptic pH(cyto) shifts cannot be dismissed as artifacts of ex vivo preparations.

Direct injection of indicators for calcium imaging at the Drosophila larval neuromuscular junction

Calcium imaging is a technique in which Ca(2+)-binding molecules are loaded into live cells and as they bind Ca(2+) they "indicate" the concentration of free calcium through a change in either the intensity or the wavelength of light emitted (fluorescence or bioluminescence). There are several possible methods for loading synthetic Ca(2+) indicators into subcellular compartments, including topical application of membrane-permeant Ca(2+) indicators, forward-filling of dextran conjugates, and direct injection. Calcium imaging is a highly informative technique in neurobiology because Ca(2+) is involved in many neuronal signaling pathways and serves as the trigger for neurotransmitter release. This article describes the direct injection of Ca(2+) indicators at the Drosophila larval neuromuscular junction (NMJ). This technique allows rapid loading of most Ca(2+) indicators, but there are drawbacks in that it is a difficult technique to master and requires additional electrophysiological equipment. Also, Ca(2+) indicators that are easily injected are usually susceptible to compartmentalization.

Topical application of indicators for calcium imaging at the Drosophila larval neuromuscular junction

Calcium imaging is a technique in which Ca(2+)-binding molecules are loaded into live cells and as they bind Ca(2+) they "indicate" the concentration of free calcium through a change in either the intensity or the wavelength of light emitted (fluorescence or bioluminescence). There are several possible methods for loading synthetic Ca(2+) indicators into subcellular compartments, including topical application of membrane-permeant Ca(2+) indicators, forward-filling of dextran conjugates, and direct injection. Calcium imaging is a highly informative technique in neurobiology because Ca(2+) is involved in many neuronal signaling pathways and serves as the trigger for neurotransmitter release. This article describes the topical application of Ca(2+) indicators at the Drosophila larval neuromuscular junction (NMJ). This loading technique is simple to execute and yields data quickly. The drawback is that the data can be difficult to interpret, primarily because it is difficult to ascertain which cellular and subcellular compartment(s) are loaded (e.g., muscle, nerve, or glia; cytosol, mitochondrion, or endoplasmic reticulum).

Forward-filling of dextran-conjugated indicators for calcium imaging at the Drosophila larval neuromuscular junction

Calcium imaging is a technique in which Ca(2+)-binding molecules are loaded into live cells and as they bind Ca(2+) they "indicate" the concentration of free calcium through a change in either the intensity or the wavelength of light emitted (fluorescence or bioluminescence). There are several possible methods for loading synthetic Ca(2+) indicators into subcellular compartments, including topical application of membrane-permeant Ca(2+) indicators, forward-filling of dextran conjugates, and direct injection. Calcium imaging is a highly informative technique in neurobiology because Ca(2+) is involved in many neuronal signaling pathways and serves as the trigger for neurotransmitter release. This article describes the forward-filling of dextran-conjugated indicators at the Drosophila larval neuromuscular junction (NMJ). This technique is particularly well suited for imaging changes in cytosolic Ca(2+) as dextran conjugation prevents compartmentalization of the Ca(2+) indicator. The major drawback is that the nerves must be severed at the start of the loading process, several hours before nerve terminals are ready to examine.

Imaging and analysis of nonratiometric calcium indicators at the Drosophila larval neuromuscular junction

Ca(2+) indicators can be loaded into a Drosophila larval neuromuscular junction (NMJ) preparation using several methods, including topical application of membrane-permeant Ca(2+) indicators, forward-filling of dextran conjugates, and direct injection. This article describes how such an NMJ preparation loaded with Ca(2+) indicator is set up for imaging of the muscle fiber during stimulation of its innervating nerve cell. A simple protocol is provided for collecting and analyzing a set of imaging data, together with the sequence of calculations involved in image analysis. The change in the intensity of the Ca(2+) indicator must be quantified to obtain an estimate of the change in the concentration of free Ca(2+) (Δ[Ca(2+)]). The change in intensity is conventionally represented as the expression "ΔF/F." Simply put, this is the change in fluorescence intensity relative to the resting fluorescence intensity. If the K(D) of the Ca(2+) indicator is in excess of the maximum value of [Ca(2+)] during the response, then ΔF/F is considered to be linearly related to Δ[Ca(2+)]. In practice, ΔF/F is calculated for each image using a simple algorithm ([F(stim) - F(rest)]/F(rest)), where F(stim) is the intensity of the Ca(2+) indicator in each image, and F(rest) is the intensity before nerve stimulation. Finally, various options for building a Ca(2+)-imaging rig are considered.

Calcium imaging at the Drosophila larval neuromuscular junction

Calcium imaging uses optical imaging techniques to measure the concentration of free calcium [Ca(2+)] in live cells. It is a highly informative technique in neurobiology because Ca(2+) is involved in many neuronal signaling pathways and serves as the trigger for neurotransmitter release. The technique relies on loading Ca(2+) indicators into cells, measuring the quantity and/or wavelength of the photons emitted by the Ca(2+) indicator, and interpreting these data in terms of [Ca(2+)]. There are several possible methods for loading synthetic Ca(2+) indicators into subcellular compartments, for example, topical application of membrane-permeant Ca(2+) indicators, forward-filling of dextran conjugates, and direct injection. These techniques are applicable to calcium imaging at the Drosophila larval neuromuscular junction (NMJ), and are also readily adaptable to Drosophila embryo and adult preparations.

Calcium-dependent phosphorylation regulates neuronal stability and plasticity in a highly precise pacemaker nucleus

Specific types of neurons show stable, predictable excitability properties, while other neurons show transient adaptive plasticity of their excitability. However, little attention has been paid to how the cellular pathways underlying adaptive plasticity interact with those that maintain neuronal stability. We addressed this question in the pacemaker neurons from a weakly electric fish because these neurons show a highly stable spontaneous firing rate as well as an N-methyl-D-aspartate (NMDA) receptor-dependent form of plasticity. We found that basal firing rates were regulated by a serial interaction of conventional and atypical PKC isoforms and that this interaction establishes individual differences within the species. We observed that NMDA receptor-dependent plasticity is achieved by further activation of these kinases. Importantly, the PKC pathway is maintained in an unsaturated baseline state to allow further Ca(2+)-dependent activation during plasticity. On the other hand, the Ca(2+)/calmodulin-dependent phosphatase calcineurin does not regulate baseline firing but is recruited to control the duration of the NMDA receptor-dependent plasticity and return the pacemaker firing rate back to baseline. This work illustrates how neuronal plasticity can be realized by biasing ongoing mechanisms of stability (e.g., PKC) and terminated by recruiting alternative mechanisms (e.g., calcineurin) that constrain excitability. We propose this as a general model for regulating activity-dependent change in neuronal excitability.

Modification of a hydrophobic layer by a point mutation in syntaxin 1A regulates the rate of synaptic vesicle fusion

Both constitutive secretion and Ca²⁺-regulated exocytosis require the assembly of the soluble N-ethylmaleimide–sensitive factor attachment protein receptor (SNARE) complexes. At present, little is known about how the SNARE complexes mediating these two distinct pathways differ in structure. Using the Drosophila neuromuscular synapse as a model, we show that a mutation modifying a hydrophobic layer in syntaxin 1A regulates the rate of vesicle fusion. Syntaxin 1A molecules share a highly conserved threonine in the C-terminal +7 layer near the transmembrane domain. Mutation of this threonine to isoleucine results in a structural change that more closely resembles those found in syntaxins ascribed to the constitutive secretory pathway. Flies carrying the I254 mutant protein have increased levels of SNARE complexes and dramatically enhanced rate of both constitutive and evoked vesicle fusion. In contrast, overexpression of the T254 wild-type protein in neurons reduces vesicle fusion only in the I254 mutant background. These results are consistent with molecular dynamics simulations of the SNARE core complex, suggesting that T254 serves as an internal brake to dampen SNARE zippering and impede vesicle fusion, whereas I254 favors fusion by enhancing intermolecular interaction within the SNARE core complex.

Most living cells constantly renew their membrane compositions and frequently communicate with neighboring cells by delivering cargo molecules from small vesicles. A key step in cargo delivery requires the fusion of the vesicle membrane with the target membrane mediated by SNARE proteins. In most cellular compartments, fusion occurs constitutively, requiring little participation of other molecules. In other cellular compartments, such as synapses in the nervous system, vesicle fusion is predominantly triggered by intracellular calcium ions. At present, constitutive and regulated fusion modes are not well understood.

In this study, we found that a mutant SNARE protein, syntaxin at the synapse, contained a building block commonly conserved for syntaxins functioning along constitutive secretory pathways. Further, our modeling predicted that the mutant syntaxin could form a tightly packed SNARE bundle closely resembling that found in the endosome, but differing from the relatively loosely packed bundle found at the wild-type synapse. Our experimental data support the hypothesis that the mutant syntaxin lowered the energy barrier for vesicle fusion by tightening the SNARE bundle. These findings reveal a novel, intrinsic structural feature of the SNARE complex that regulates vesicle fusion rate at different cellular compartments.

A syntaxin 1A threonine to isoleucine mutation is found to enhance SNARE complex formation and vesicle fusion. This structural change results in a syntaxin that resembles those in constitutive secretory pathways.

Loading Drosophila nerve terminals with calcium indicators

Calcium plays many roles in the nervous system but none more impressive than as the trigger for neurotransmitter release, and none more profound than as the messenger essential for the synaptic plasticity that supports learning and memory. To further elucidate the molecular underpinnings of Ca(2+)-dependent synaptic mechanisms, a model system is required that is both genetically malleable and physiologically accessible. Drosophila melanogaster provides such a model. In this system, genetically-encoded fluorescent indicators are available to detect Ca(2+) changes in nerve terminals. However, these indicators have limited sensitivity to Ca(2+) and often show a non-linear response. Synthetic fluorescent indicators are better suited for measuring the rapid Ca(2+) changes associated with nerve activity. Here we demonstrate a technique for loading dextran-conjugated synthetic Ca(2+) indicators into live nerve terminals in Drosophila larvae. Particular emphasis is placed on those aspects of the protocol most critical to the technique's success, such as how to avoid static electricity discharges along the isolated nerves, maintaining the health of the preparation during extended loading periods, and ensuring axon survival by providing Ca(2+) to promote sealing of severed axon endings. Low affinity dextran-conjugated Ca(2+)-indicators, such as fluo-4 and rhod, are available which show a high signal-to-noise ratio while minimally disrupting presynaptic Ca(2+) dynamics. Dextran-conjugation helps prevent Ca(2+) indicators being sequestered into organelles such as mitochondria. The loading technique can be applied equally to larvae, embryos and adults.

Synaptic homeostasis on the fast track

Synaptic homeostasis is a phenomenon that prevents the nervous system from descending into chaos. In this issue of Neuron, Frank et al. overturn the notion that synaptic homeostasis at Drosophila NMJs is a slow developmental process. They report that postsynaptic changes are offset within minutes by a homeostatic increase in neurotransmitter release that requires the presynaptic Ca(2+) channel Cacophony.

TheDrosophila cacts2mutation reduces presynaptic Ca2+entry and defines an important element in Cav2.1 channel inactivation

Voltage-gated Ca2+ channels in nerve terminals open in response to action potentials and admit Ca2+, the trigger for neurotransmitter release. The cacophony gene encodes the primary presynaptic voltage-gated Ca2+ channel in Drosophila motor-nerve terminals. The cac(ts2) mutant allele of cacophony is associated with paralysis and reduced neurotransmission at non-permissive temperatures but the basis for the neurotransmission deficit has not been established. The cac(ts2) mutation occurs in the cytoplasmic carboxyl tail of the alpha1-subunit, not within the pore-forming trans-membrane domains, making it difficult to predict the mutation's impact. We applied a Ca2+-imaging technique at motor-nerve terminals of mutant larvae to test the hypothesis that the neurotransmission deficit is a result of impaired Ca2+ entry. Presynaptic Ca2+ signals evoked by single and multiple action potentials showed a temperature-dependent reduction. The amplitude of the reduction was sufficient to account for the neurotransmission deficit, indicating that the site of the cac(ts2) mutation plays a role in Ca2+ channel activity. As the mutation occurs in a motif conserved in mammalian high-voltage-activated Ca2+ channels, we used a heterologous expression system to probe the effect of this mutation on channel function. The mutation was introduced into rat Ca(v)2.1 channels expressed in human embryonic kidney cells. Patch-clamp analysis of mutant channels at the physiological temperature of 37 degrees C showed much faster inactivation rates than for wild-type channels, demonstrating that the integrity of this motif is critical for normal Ca(v)2.1 channel inactivation.

Single neuron activity in the Drosophila larval CNS detected with calcium indicators

Although the Drosophila larva has been extensively used for genetic studies of synaptic transmission and locomotion, neurophysiological studies have lagged because it is difficult to investigate circuitry and synaptic function in the larval central nervous system (CNS). Here we introduce an optical technique to monitor neuronal activity in the intact Drosophila larval CNS. We loaded neurons retrogradely through cut axons with dextran-conjugated calcium indicators. Fluorescence responses to changes in the concentration of intracellular calcium are sufficiently fast and large to monitor electrical activity in single neurons. Responses to action potentials were detected in motor neuron cell bodies, axons, neurites, dendrites and sensory neuron afferents identified by genetically targeted green fluorescent protein expression. Our findings provide an experimental procedure for testing synaptic function and connectivity within the intact larval CNS.

Fast calcium signals in Drosophila motor neuron terminals

Drosophila is a powerful model for neuroscientists, but physiological techniques have not kept pace with advances in molecular genetics. We introduce a reliable assay for intracellular calcium dynamics in Drosophila larval motor neuron terminals, and a new physiological solution that improves the longevity of the larval preparation. By loading calcium indicators into motor neuron terminals through cut axons, we obtained a high signal-to-noise ratio with confocal microscopy, and good temporal resolution of calcium-dependent fluorescence changes. We provide an estimate for the resting intracellular calcium concentration, the first description of calcium kinetics for a single action potential (AP), and improved resolution of calcium kinetics during AP trains. The very rapid decay of the calcium signal following a single AP (tau ~60 ms) indicates a previously unreported fast calcium extrusion mechanism in Drosophila motor neuron terminals well suited for sustaining physiological processes during the high rates of impulse activity which drive locomotor activity.

Formation and function of synapses with respect to Schwann cells at the end of motor nerve terminal branches on mature amphibian (Bufo marinus) muscle

A study has been made of the formation and regression of synapses with respect to Schwann cells at the ends of motor nerve terminal branches in mature toad (Bufo marinus) muscle. Synapse formation and regression, as inferred from the appearance and loss of N-(3-triethylammoniumpropyl)-4-(4-(dibutylamino)styryl) pyridinium dibromide (FM1-43)-stained vesicle clusters, occurred at the ends of terminal branches over a 16 hr period. Multiple microelectrodes placed in an array about FM1-43 blobs at the ends of terminal branches detected the electrical signs of neurotransmitter being released onto receptors. Injection of a calcium indicator (Oregon Green 488 BAPTA-1) into the motor nerve with subsequent imaging of the calcium transients, in response to stimulation, often showed a reduced calcium influx in the ends of terminal branches. Injection of a fluorescent dye into motor nerves revealed the full extent of their terminal branches and growing processes. Injection of the terminal Schwann cells (TSCs) often revealed pseudopodial TSC processes up to 10-microm-long. Imaging of these TSC processes over minutes or hours showed that they were highly labile and capable of extending several micrometers in a few minutes. Injection of motor nerve terminals with a different dye to that injected into their TSCs revealed that terminal processes sometimes followed the TSC processes over a few hours. It is suggested that the ends of motor nerve terminals in vivo are in a constant state of remodeling through the formation and regression of processes, that TSC processes guide the remodeling, and that it can occur over a relatively short period of time.

Quantal potential fields around individual active zones of amphibian motor-nerve terminals

The release of a quantum from a nerve terminal is accompanied by the flow of extracellular current, which creates a field around the site of transmitter action. We provide a solution for the extent of this field for the case of a quantum released from a site on an amphibian motor-nerve terminal branch onto the receptor patch of a muscle fiber and compare this with measurements of the field using three extracellular electrodes. Numerical solution of the equations for the quantal potential field in cylindrical coordinates show that the density of the field at the peak of the quantal current gives rise to a peak extracellular potential, which declines approximately as the inverse of the distance from the source at distances greater than about 4 microm from the source along the length of the fiber. The peak extracellular potential declines to 20% of its initial value in a distance of about 6 microm, both along the length of the fiber and in the circumferential direction around the fiber. Simultaneous recordings of quantal potential fields, made with three electrodes placed in a line at right angles to an FM1-43 visualized branch, gave determinations of the field strengths in accord with the numerical solutions. In addition, the three electrodes were placed so as to straddle the visualized release sites of a branch. The positions of these sites were correctly predicted on the basis of the theory and independently ascertained by FM1-43 staining of the sites. It is concluded that quantal potential fields at the neuromuscular junction that can be measured with available recording techniques are restricted to regions within about 10 microm of the release site.

Vesicle-associated proteins and quantal release at single active zones of amphibian (Bufo marinus) motor-nerve terminals

A study was made to determine the disposition of vesicle-associated proteins (syntaxin, SV2, SNAP-25) and calcium channels with respect to the spatial extent of spontaneous and evoked quantal release within regions of amphibian motor-nerve terminal branches delineated by FM1-43 stained vesicle clusters (blobs). Discrete concentrations of vesicles revealed approximately 2 microm apart along the length of terminal branches through FM1-43 staining were identical in size and spacing to those identified along terminal branches with SV2 antibody (AbSV2). Fluorescent antibodies to syntaxin 1 (AbS), SNAP-25 (AbS25) and the calcium channel alpha1B subunit (Abalpha1B) were found in relatively high concentrations coincident with the AbSV2 blobs. Three extracellular recording electrodes were placed in the vicinity of individual FM1-43 blobs, and an algorithm was used to determine the spatial origin of miniature endplate potentials (MEPPs) and EPPs together with their relative amplitudes. MEPPs and EPPs originated throughout the region stained by FM1-43 but not elsewhere; amplitude-frequency distributions of MEPPs and EPPs were similar for all FM1-43 blobs with average coefficients of variation of no less than 0.28. A linear relationship existed between the size of an FM1-43 blob, measured as the integrated extent of FM1-43 staining of a blob, and the frequency of MEPPs as well as the probability of EPPs from the blob. There was a proximo-distal gradient in the size of FM1-43 blobs along the length of single terminal branches, suggesting a gradient in release probability along the branches. The frequency distribution of the distances between blobs was approximately Gaussian, whereas the frequency distribution of the size of blobs was highly skewed and was best fitted with a gamma distribution. It is concluded that there are correlations among the extent of labeling of SNAP-25, syntaxin and calcium channels at a release site, the store of vesicles to be found there, and the probability of spontaneous and evoked quantal release.

Quantal secretion and nerve-terminal cable properties at neuromuscular junctions in an amphibian (Bufo marinus)

The effect of a conditioning depolarizing current pulse (80-200 micros) on quantal secretion evoked by a similar test pulse at another site was examined in visualized motor-nerve terminal branches of amphibian endplates (Bufo marinus). Tetrodotoxin (200 nM) and cadmium (50 microM) were used to block voltage-dependent sodium and calcium conductances. Quantal release at the test electrode was depressed at different distances (28-135 microm) from the conditioning electrode when the conditioning and test pulses were delivered simultaneously. This depression decreased when the interval between conditioning and test current pulses was increased, until, at an interval of approximately 0.25 ms, it was negligible. At no time during several thousand test-conditioning pairs, for electrodes at different distances apart (28-135 microm) on the same or contiguous terminal branches, did the electrotonic effects of quantal release at one electrode produce quantal release at the other. Analytic and numerical solutions were obtained for the distribution of transmembrane potential at different sites along terminal branches of different lengths for current injection at a point on a terminal branch wrapped in Schwann cell, in the absence of active membrane conductances. Solutions were also obtained for the combined effects of two sites of current injection separated by different time delays. This cable model shows that depolarizing current injections of a few hundred microseconds duration produce hyperpolarizations at approximately 30 microm beyond the site of current injection, with these becoming larger and occurring at shorter distances the shorter the terminal branch. Thus the effect of a conditioning depolarizing pulse at one site on a subsequent test pulse at another more than approximately 30 microm away is to substantially decrease the absolute depolarization produced by the latter, provided the interval between the pulses is less than a few hundred microseconds. It is concluded that the passive cable properties of motor nerve terminal branches are sufficient to explain the effects on quantal secretion by a test electrode depolarization of current injections from a spatially removed conditioning electrode.

Probability of quantal secretion and the mobilization of vesicles at the active zones of endplates

A vesicle mobilization model is developed for active zones of motor-nerve terminals. In the model, vesicles from a storage pool are recruited to an available pool where they are docked at the active zone, the latter pool being replenished by endocytosis. The predictions of this model are then compared with experimental results on these active zones. If a styryl dye (FM1-43) is used to stain all of the vesicles at motor-nerve terminals then the stores of vesicles associated with different active zones are shown to vary considerably in size. Under the assumption that the available pool size is proportional to the storage pool size, the mobilization model shows that the extent of destaining of these stores during a particular period of stimulation is proportional to the size of the storage pool, this size being indicated by the initial staining. This is not so under the alternative assumption that the available pool size does not vary proportionally with store size. If the two pools are proportional then the number of quanta released from the available pool is proportional to the storage pool size. Experimental results indicate that there is a linear relationship between destaining and staining, as well as between quantal release and staining at different sites along motor-nerve terminals, thus favouring the hypothesis that available pool size (or the size of the active zone) and store size are proportional. Given that there are very different store sizes along the length of motor-nerve terminals, this analysis provides further evidence for the non-uniform probability of quantal secretion at different active zones within a single motor-nerve terminal.

Calcium dependence of quantal secretion from visualized sympathetic nerve varicosities on the mouse vas deferens

1. The effects of calcium on the secretion of quanta recorded from single varicosities on the surface of the mouse vas deferens has been determined. 2. If recordings were made from two adjacent varicosities and the [Ca2+]o increased from a low value (1 mM), then the increase in the mean quantal content of the endplate potential (m(e)) was almost entirely due to an increase in the binomial probability for secretion (pe). At higher [Ca2+]o there was an increasing tendency for the binomial parameter (ne) to increase from 1 to 2. When ne increased there was very little change in pe, indicating that the new release site recruited from the other varicosity has a relatively low probability for secretion. 3. If recordings were restricted to single varicosities and the [Ca2+]o increased in the range from 1 to 4 mM then the increases in m(e) were almost always due to an increase in pe. The gradient relating the log of m(e) to the log of [Ca2+]o for [Ca2+]o of 1.0-1.5 mM was in the range 3.2-5.4 (mean 4.2). 4. Test impulses gave a similar proportional increase in m(e) following a conditioning impulse at all varicosities from which recordings were made. 5. Facilitation of m(e) declined exponentially with an increase in the test-conditioning interval from 0.5 to 4 s. The time constant for this decline was about 6 s. 6. The results indicate that single release sites show a similar fourth power dependency on [Ca2+]o and facilitate to about the same degree following a conditioning impulse.